Composition and process for glazing ceramic ware

ABSTRACT

THIS PATENT APPLICATION DESCRIBES A COMPOSITION FOR GLAZING CERAMIC WARE, PREPARATIONS USEFUL FOR MAKING SAME, PROCESS FOR GLAZING BODIES AND SUBSTRATES WITH SAME, AND RESULTING COATED CERAMIC BODIES. THE COMPOSITION FOR GLAZING IS CHARACTERIZED BY CONTAINING A PARTICULATE VITRIFIABLE MATERIAL, AT LEAST A SUBSTANTIAL FRACTION OF WHICH IS IN THE PARTICULATE VITREOUS STATE, SAID VITRIFIABLE MATERIAL, AFTER MELTING INTO A FLUENT VITREOUS STATE, BEIG SELF-NUCLEATING OR AUTOCRYSTALLIZABLE OR CRYSTALLIZABLE INTO A SUBSTANTIALLY DIMENSIONALLY STABLE CONTINOUS VITREOUS FILM IN WHICH ARE DISPERED CRYSTALS OF LOW THERMAL EXPANSION. THE GLAZING COMPOSITION IS PARTICULARLY SUITED FOR GLAZING LOW EXPANSION CERAMIC WHITEWARE. SET FORTH AS USEFUL PREPARATIONS. ARE SPECIAL PARTICULATE LITHIA-ALUMINA-SILCIA AND ALKALINE EARTH-ALUMINA-SILICA SYSTEMS MODIFIED WITH FLUX IN PROPORTION CONTROLLED TO RESTRICT THE DEVELOPMENT OF A PRIMARY LOW THERMAL EXPANSION CRYSTALLINE PHASE, SOME OF SAID PREPARATIONS BEING MODIFIED WITH ZIRCONIA. THE GLAZING PROCESS COMPRISES PARTIALLY COATING A CERAMIC BODY WITH THE COMPOSITION FOR GLAZING, FIRING THE COATED BODY AT A TEMPERATURE SUFFICIENTLY HIGH AND FOR A TIME SUFFICIENTLY LONG FOR CONVERTING THE COMPOSITIO INTO A FLUENT CONTINUOUS VITREOUS SURFACE COATING, THEN ADJUSTING THE TEMPERATURE TO A VALUE AT WHICH CRYSTAL GROWTH IN THE SURFACE COATING OCCURS AT A MEASURABLE RATE, AND FINALLY COOLING THE RESULTANT GLAZED WARE AT A RATE CONSISTENT WITH KEEPING THE WARE INTEGRAL. THE INVENTION SHOWS PARTICULAR ADVANTAGE FOR MAKING GLAZES OF ADJUSTABLE AND LOW THERMAL EXPANSION FOR THERMAL SHOCK AND MECHANICAL SHOCK RESISTANT CERAMIC WHITEWARE, FOR EXAMPLE, DINNERWARE, COOKWARE, CERAMIC TILE, ACOUSTICAL TILES OF THE MINERAL TYPE, SANITARY WARE, ARTWARE AND ELECTRICAL AND TECHNICAL PORCELAIN. THE INVENTION IS SPECIALLY ADAPTABLE TO CONVENTIONAL &#34;TWO FIRE&#34; GLAZING PRACTICE.

United States Patent Oflice 3,679 464 COMPOSITION AND PROCESS FOR GLAZING CERAMIC WARE Richard Andrew Eppler, Timonium, Md., assignor to SCM Corporation, Cleveland, Ohio No Drawing. Application May 27, 1969, Ser. No. 828,345, now Patent No. 3,561,984, dated Feb. 9, 1971, which is a continuation-in-part of application Ser. No. 625,000, Mar. 22, 1967. Divided and this application Oct. 21, 1970, Ser. No. 82,803

Int. Cl. C03c 25/00 US. Cl. 117-175 6 Claims ABSTRACT OF THE DISCLOSURE This patent application describes a composition for glazing ceramic ware, preparations useful for making same, process for glazing bodies and substrates with same, and resulting coated ceramic bodies. The composition for glazing is characterized by containing a particulate vitrifiable material, at least a substantial fraction of which is in the particulate vitreous state, said vitrifiable material, after melting into a fluent vitreous state, being self-nucleating or autocrystallizable or crystallizable into a substantially dimensionally stable continuous vitreous film in which are dispersed crystals of low thermal expansion. The glazing composition is particularly suited for glazing low expansion ceramic whiteware. Set forth as useful preparations are special particulate lithia-alumina-silica and alkaline earth-alumina-silica systems modified with flux in proportion controlled to restrict the development of a primary low thermal expansion crystalline phase, some of said preparations being modified with zirconia. The glazing process comprises partially coating a ceramic body with the composition for glazing, firing the coated body at a temperature sufliciently high and for a time sufliciently long for converting the composition into a fluent continuous vitreous surface coating, then adjusting the temperature to a value at which crystal growth in the surface coating occurs at a measurable rate, and finally cooling the resultant glazed ware at a rate consistent with keeping the ware integral. The invention shows particular advantage for making glazes of adjustable and low thermal expansion for thermal shock and mechanical shock resistant ceramic whiteware, for example, dinnerware, cookware, ceramic tile, acoustical tiles of the mineral type, sanitary ware, artware and electrical and technical porcelain. The invention is specially adaptable to conventional two-fire glazing practice.

This application is a division of copending application S.N. 828,345 filed May 27, 1969, now US. Pat. 3,561,984 issued on Feb. 9, 1971 which is a continuation-in-part of application S.N. 625,000 filed Mar. 22, 1967 (now abandoned), the disclosures of which are incorporated herein by reference.

This invention relates to a composition for glazing ceramic ware, preparations useful for making said composition, process for glazing ceramic bodies and substrates with said composition, and the resulting coated ceramic bodies.

Herctofore compositions for glazing ceramic ware have not been available with the extremely low coeflicients of thermal expansion possible by using this invention. A particular advantage of this invention is that it is specially adaptable to conventional glazing practice such as the US. two-fire glazing practice as well as other techniques. The invention is also particularly advantageous for making glazes of adjustable and very low thermal expansion for thermal shockand/or mechanical shock-resistant ceramic whiteware, for example, dinnerware, cookware,

ceramic tile, acoustical tiles of the mineral type, sanitary ware, art ware, and electrical and technical porcelain.

In one aspect the invention is a composition for glazing ceramic ware comprising 100 parts of particulate vitrifiable material, at least a substantial fraction of which initially is in the vitreous state, said vitrifiable material, after melting into a fluent vitreous state, being partially devitrifiable and crystallizable into a substantially dimensionally stable continuous, vitreous coating in which are dispersed crystals having average coeflicient of thermal expansion less than 4x 10 C.; 0-15 parts ceramic clay (other than a montmorillonite clay); 0-5 parts of a montmorillonite (such as bentonite) as a suspending assistant; 0-20 parts colorant or stain; 0-20 parts ceramic opacifier such as zirconia, titania, tin oxide, or cerium oxide; 0-0.4 part electrolyte for suspending, dispersing, peptizing and/or thickening such as calcium chloride, sodium silicate, sodium tetrapyrophosphate, methyl cellulose and sodium carboxy methyl cellulose; and 0-90 parts water.

Typical water concentrations in a" resulting slip would be between about 15 and about parts per parts of the particulate vitrifiable material, for example, 20-40 parts for a dipping application of the glazing composition; 70-90 parts for roller coat application; and 60-80 parts for spray application. Other forms of application can include waterfall glazing of the ceramic body or substrate, or brushing or blade application of such slip.

The particulated vitrifiable material useful for partially devitrifying or crystallizing into the low expansion glaze having a continuous vitreous phase should be at least partially in the vitreous state initially. Suitably at least the water soluble constituents such as carbonates, borax, boric acid, sodium fluoride or potassium fluoride should be combined as vitreous matter so as to resist water solution. It is especially desirable to have no more than one percent by weight of the particulate devitrifiable material extractable in water at room temperature when 100 grams of the material are suspended in 1000 ml. of water. Advantageously, the particulate vitrifiable material is at least 1045 percent by weight in the vitreous state and even higher to achieve best interaction of all the components in in subsequent firing and resulting glazing. For example, the particulate vitrifiable material can be petalite mixed with other materials which are entirely in the vitreous state as a frit or a mixture of frits. Preferably the entire particulate vitrifiable material in the composition other than the listed mill additions is a frit or a mixture of frits. The optional stains, opacifiers, clay, bentonite and other solids can be admixed intimately into the composition as mill additives. Wet grinding of the composition ordinarily is practiced, but the solids can be ground dry if desired, with water added subsequently to the premilled solids to make a slip. To achieve best utility and performance there should be no more than about 3 percent, and preferably no more than about /2 percent of the particles retained on a 325 mesh (Tyler Standard) screen after milling of the composition for application to a ceramic body.

The particularly low average coeflicients of thermal expansion of the crystals formed when the particulate vitrifiable material is partially devitrified or crystallized is the basis for the special utility of the instant composition for glazing ceramic ware. These crystals formed in the devitrification should have average thermal expansion less than 4 l0- C., suitably no more than about 2.5 X10- down to about 1x 10 or even no measurable coefiiicent or a slightly negative one such as minus l 10 Average thermal expansion of a crystal means that the expansion is averaged over all crystallographic directions. These crystals are held in a matrix of continuous vitreous coating to yield a substantially non-porous glaze over the ceramic substrate, which glaze can be made glossy or matte Patented July 25, 1972 as necessary or desired for utility in accordance with invention principles. The overall glaze composition after firing should have an overall coetficient of thermal expansion less than 5Xl0- C., advantageously about 1X10- to 4X10-/ C-., and preferably about l- 10- to 3 l0-/ C. The thermal expansion values spoken of in'this specification in connection with glazes and bodies are the average linear thermal'expansion coefficientof these materials for the temperature range of 50-35()" C. In order to obtain the glazing effectiveness, the particulatedevitrifiable material must be meltable into a fluent vitreous state of comparatively low viscosity for flowing and spreading onto the ware and giving the glaze film.

. The particulate vitn'fiable material of the composition can ful'in compounding the composition for glazing from the composition as a whole, which can include various mill additions, said' particulate vitrifiable material by itself hereafter will be referred to as a preparation. In another respect of this invention, particularly useful prep- 7 arations are set forth which yield upon firing the required fluency for glazing and subsequently are crystallizable or divitrifiable into a dimensionally stable continuous vitreous coating in which are dispersed the very low expasion crystals. The flux (which can be considered a diluent) reacts to make a glass of the preparation in the firing operation, the flux being fully soluble in the fluent melt.

The low expansion crystals formed on the ensuing crystallization or devitrification in the lithium-bearing preparations of this invention are primarily lithium alumino-silicate crystalline structures and can be the stuffed .quartz structure as described in the Beall US. Pat. 3,252,- 811, p-eucryptite (Li,O-Al O -2SiO and/or fi-spodumene Li,0-A1,0,-4Si0,) (which has a coeflicient not as low as that of stuffed quartz and therefore not considered as useful in all aspects of the invention.) 7 The composition of the uncrystallized preparation must be such as to thermally crystallize to the appropriate type and amount of low expansion lithium aluminosilicate phasesto form a semicrystalline glaze having a composite or overall] thermal expansion coeflicient of less than 5X 10 C(To insure the in situ crystallization of the appropriate lithiumaluminosilicate crystalline phases in the glaze, the alumina (A1 content of the uncrystallized preparation is quite critical.

According to the present invention, the glazing preparations must contain at least 17% by weight of alumina (A1 0 to achieve these desired low expansions. When the A1 0 content is less than 17% by weight the expansion of the resulting semicrystalline glaze is higher than X10 C. A minimum of 17% A1 0 is therefore required to obtain preparations which are thermally, in situ, autocrystallizable to form low expansion, semicrystalline, glass-ceramic glazes having a coefficient of thermal expension of less than 5X C. The thermal expansion of this semicrystalline glaze is a weighted mean of the expansion of the uncrystallized glassy matrix (relatively high expansion) and the expansion of the lithium aluminosilicate crystalline phase (relatively low expansion).

The reason for this unexpected criticality of A1 0 is not presently fully understood, although it is suspected that when the A1 0 content is less than 17% by weight, the crystallizing vitreous phase becomes alumina deficient as the crystallization progresses, and crystallization of the higher expansion lithium metasilicate (Li O-SiO and lithium disilicate (Li O-2SiO crystalline phases occur at the expense of the low expansion lithium aluminosilicate phases. Regardless of the crystallization mechanism responsible, it can be stated that preparation compositions containing at least 17% by weight of A1 0 crystallize to pansion in the range of about ll0 10-' C. which are compatible with high expansion metallicsubstrates such as mild steel. The crystalline phases present are. described as lithium titanium silicates having thermalex- V pansion coefiicients of about -90X 10- C. This pres: ence of these high expansion crystalline phases result in semicrystalline enamels having coeflicients of expansion of f 75 x l0- C. and higher which is unacceptable for use as a low expansion glaze. For instance, enamels 7, 9 and 11 from Table II of the Sanford patent contain about 15-16% alumina. These enamels are unsuitable for use as low expansion whiteware glazes because enamel 7 has a coefficient of expansion of l0*"/ C., enamel 9 d whitewares because nucleants often detract from the color 4 has a coefficient of expansion of 1 00 l0-"/ C. and

enamel 11 has a coeflicient of expansion of X10-"/ According to another feature of the present invention,"

it is unnecessary to incorporate a nucleant into the prep- 7 aration to induce the nucleation or growth of the low expansion lithium aluminosilicate crystalline phases during the subsequent heat treatment. The amounts and proportions of Lip-A1 0 and SiO, in the preparation are specified so that the appropriate low expansion phases are self-nucleating or autocrystallizable under the influence of the energy supplied by the specified heat treatment.

This is an important advantage in the glazing of ceramic and appearance of the finished glazed product. For instance, titania (TiO is a commonly employed nucleant in crystallizable glazes. Unfortunately, the presence of this titania nucleant tends to cause a mottled appearance as well as a brown discoloration in the fired glaze. Mottling and brown discoloration are undesirable and commercially unacceptable for many whiteware applications. Additionally, titania nucleated glazes are often very low in gloss and have a dull or matte appearance. Self-nuclep ated glazes of the present invention can be bright, white or ivory in color and glossy. I

Low expansion lithium aluminosilicate glazes containing titania nucleants have been studied in the past. For instance, see the article Studies on the Glazes of Lithia Ceramics by Maki and Tashiro, published in the Journal of the Ceramic Associationof Japan, volume 74, No. 3,

pages 89-93, 1966, wherein a glazing composition containing 50.4% SiO ,'29.2% A1203, 5.9% Li O, 1.7% Zl'Og, P205, Tiog, N320, K30, 2.8% B 0 and 2.8%'Pb0 is disclosed. While this composition does form a low expansion glaze, the glaze is mottled, dull or matte and has a yellow-brown discoloration which is not particularly desirable in whiteware applications where bright, white or ivory glossy surfaces are required. When this glazing composition is prepared and applied to a petalite body as disclosed in the article, the

titania nucleants immediately form a slight yellow discoloration. This yellow discoloration becomes brownish-- the temperature is raised to about 750 to 1000 C. to

promote crystalline growth to form the semicrystalline ceramic glaze. The present compositions and processes eliminate the need for nucleating agents such as titania and also eliminates the need for the nucleation heat treatment. Accordingly, bright, white (non-yellow), glossy, ceramic whiteware glazes are presently attainable.

The crystalline phases discussed above are identified by X-ray diffraction analysis and accordingly the designation of the crystalline phases as stuffed quartz, fl-eucryptite and fl-spodumene includes low expansion lithium aluminosilicate crystalline phases which are identified as stuffed quartz, p-eucryptite or p-spodumene :by X-ray diffraction.

In preparations involving BaO-Al O -SiO the low expansion celsian crystalline phase can precipitate. In preparations involving BaO-MgO-AlgOySiO systems an unnamed low expansion crystal can precipitate, this crystal approximating one equivalent of barium oxide and two of magnesium oxide per five equivalents of alumina and 12 equivalents of silica; it is referred to in the text, Thermal Properties of Ceramics, page 29, by Smoke and Koenig (1958). In preparations involving the low expansion crystals nepheline and sometimes celsian can precipitate.

Accordingly, the inventive preparations yielding the resulting low expansion glaze containing the aforementioned low expansion crystals can be described as particulate mixtures providing a special resulting ingredient composition set. The broad set based on lithium oxide is designated Set I, and is described as follows:

Ingredient: Percent Li O -23.

MgO 0-17, 0.74 part of MgO replacing 1 part of H 0 when MgO is used, the subtotal sum of Li O and MgO' being at least 3%.

Si0 36-78, the subtotal sum of the Li 0, MgO, A1 0 and SiO being 70-95%.

ZrO 0-5.

Flux -30.

wherein said flux is: B 0 K 0, F, PbO, Na O, CaO, SrO, ZnO, BaO up to 5%, or a mixture of same, and wherein the subtotal sum of the Zr-O plus said flux is 530%.

lAdvantageously for most practical operation and lower thermal expansion of the resulting glaze, the range of ingredients in Set I is restricted as follows to give Set Ia.

Ingredient: Percent Li O 4-23.

MgO 0-6, 0.74 part of MgO replacing 1 part of Li O when MgO' is used.

Si0 36-74, the subtotal sum of the Li O, MgO, A1 0 and Si0 being 80-95%.

ZrO 0-5.

Flux 5-20.

wherein the subtotal sum of the ZrO plus flux is 5-20%.

Preferably, for obtaining the most practical low expansion glazes useful in a wide variety of operations and meeting the many operational requirements in the ceramic industry for cookware and dinnerware manufacture, Set I is further restricted to Set Ib as follows:

6 Ingredient: Percent Li o 7-16. MgO 0-2 /2, 0.74 part of MgO replacing 1 part of Li O when MgO is used. A1 0 17-33. SiO 48-63, the subtotal sum of the Li O, MgO, A1 0 and Si0 being 88-95%. Zr0 0-3. Flux 5-12.

and wherein the subtotal sum of the ZrO plus flux is 5-12%.

It will be noted in the above composition sets that zirconia can be added in the proportions specified as a crystallization rate promoter or catalyst. ZrO crystallizes from the glaze as a minor phase in the form of a cubic crystal. -In some preparations, particularly those yielding the lower thermal expansion coeflicients, the presence of such minor phase yields a less glossy finish, whereas in other preparations, such as those approaching the maximum limit of overall thermal expansion herein, such minor phase imparts some additional glossiness to the resulting fired glaze.

Another inventive preparation yielding on firing the required continuous vitreous phase containing the very low expansion crystals in the necessary resulting glaze is the preparation designated herein as providing the resulting ingredient composition Set II as follows:

Ingredient: Percent BaO 5-65, but not less than 15% when no MgO or Na O is present.

MgO 0-12.

Na 0 0-15, the subtotal sum of the BaO plus MgO plus Na o being from 12% to 65%.

SiO 18-65, the subtotal sum of the B210, MgO, N330, A1203 and being 70-95%.

Flux 5-30.

wherein said flux is: B 0 K 0, F, PbO, CaO, SrO, ZnO or a mixture of same; and wherein ingredients of said preparation set are vitrified to an extent at least sufiicient for rendering said preparation substantially water-resistant.

For advantageous practical operation, Set II can be restricted further as follows, designated as Set IIa:

(Ila) Ingredient: Percent BaO 5-15.

MgO 5-10.

Na O 0-5, the subtotal sum of the BaO plus MgO plus Na O being from 12 to 25%.

SiO 45-65, the subtotal sum of the BaO, MgO, Na O, A1 0 and SiO; being -95%.

Flux 5-20.

In all instances the ingredients in the foregoing preparations are vitrified to an extent at least suflicient for rendering such preparation substantially water resistant as hereinbefore described (so that application of the prep arations to a substrate cannot involve an aqueous leaching away of the necessary interacting materials and resulting disproportionation of the preparation). Because some of the probable reactions that occur upon firing are solid state reactions, extremely intimate and substantially homogeneous distribution of the reactants in close proximity to each other certainly is desirable for greatest reaction efliciency. Accordingly, it is advantageous that a substantial fraction of the preparation be invitreous condition and preferably that the preparation is a single frit or an intimate mixture of frits to insure intimacy of the interacting components 'as well as water resistance.

In afurther aspect of the invention there is presented a process for glazing a ceramic body which comprises:

(a) at least partially coating thebody with a composition for glazing of the type described herein; (b), firing the resulting coated body at a temperature sufficiently high and for a time sufiiciently long for co'nvetting said composition into a fluent, continuous, vitreous surface coating;

(c) then adjusting temperature to a value at which crystal growth in said vitreous surface coating occurs within not substantially more than several hours;

7 (d) then cooling the resultant glazed ware at a rate consistent with keeping the ware integral.

ing. At the liquidus crystals can be present which prevent the necessary fluidity. The instantaneous fluid viscosity of the composition during the firing step shall be less than the softening point (as defined by the conventional ASTM procedure applied to glasses) which viscosity at softening point is about 10 poises. For compositions for glazing utilizing Sets I, Ia, or Ib, a useful firing temperature is from about 975-1200 C.; for compositions for glazing utilizing compositions II and 11a preferred firing temperatures are 1100-l200 C.

The most practical procedure for adjusting temperature to a value at which crystal growth in the fluent vitreous coating occurs in a practical time, that is, in several minutes to at least within several hours, is to reduce the temperature after the firing step to a temperature below the liquidus of the highest melting crystal to be produced in the resulting vitreous matrix, although in some systerns the temperature conceivably could be raised to obtain devitrification at the desired rate of crystal growth. As a practical matter the temperature stage for inducing crystal growth using Sets I, la and Ib is maintained at 650-850' C. and, using Sets II and Ha, at 700l000 C. For each particular preparation and mill additions thereto it should be understood, however, that there will be optimum temperatures to achieve this devitrification at a desired practical rate in a time not substantially more thanseveral, i.e., 20-30 hours, and usually in a much shorter overall time, e.g., as little as 8-10 minutes, to accommodate rapid production of glazed ware.

The final step in the glazing process is to cool at a rate" that is not so fast as to. cause undesirable .crackingor crazing or undesirable strains in the resulting glazed obje ct or its glazed surface, in other words, at a rate consistent with keeping the ware integral. The cooling can be done reasonably fast for production purposes, some systems being air-coolable from a temperature as high as about 816 C. to room temperature with the coated'work merely maintained on a metal support. A practical rate of cooling is one to ten degrees C. per minute, and the cooling can be quite slow where long cooling times can be accommodated.

The instant invention is to be distinguished from conventional operations for making semi-crystalline glazes for artistic purposes because such conventional glazes contain crystals having average coefficient of thermal expansion above 4X10/ C. Such glazes are not useful for glazing high petalite or cordierite bodies or other bodies of similarly low thermal expansion.

' particles.

Furthermore, the instant invention is to be distinguished from the conventional preparation of devitrified glass bodies typified by the well-known Pyroceram product and related processes (the term Pyroceram being a trademark of Corning Glass Works). Typically, in the Prep aration of such divitrified glass forms, the formulation and operation is directed to induce crystallization in glass at a fairly low temperature so as to maintain dimensional stability of the form. Practically a minimum viscosity of 10 poises is required; preferably a minimum viscosity of 10 poises should be used. In contrast, the present invention is directed to obtain working fluency of the molten devitrifiable material so it will coat a body efficiently, the viscosity in this condition being typically several powers of 10 less than the glass in the conventional glass devitrifying operation. Fluid viscosity of a typical inventive composition at useful firing temperatures was measured at,

present, then reduce temperature for the crystal growing. step. In contrast, the normal heating path for making low expansion devitrified bodies, e.g.,-as shown in U.S. Pats. 3,006,775; 3,252,811; 2,920,921; 3,161,528 and 3,272,610 is to make a glass body in the conventional sense, the body being formed in desired shape so that it isextremely highly viscous and dimensionally stable, give it a comparatively low temperature nucleating soak to maximize the viscosity within the body, then raise the temperature of the body to a temperature which is below the liquidus of the crystals being formed. U.S. Pat. 3,084,053 first converts the glass to frit, aggregates the frit into desired shape, then treats similarly.

The most suitable compositions'for glazing according to this process are those containing particulate vitrifiable material providing one of the resulting ingredient composition sets described above, that is, Set I or Set H, advantageously Set Ia and Set Ila, and preferably Set Ib, and most preferably that set which is made up so as to be substantially entirely in the vitreous state as frit The coating step preparatory to firing of the coated ceramic body can be done as previously described in connection with the composition for glazing, above, by spraying, dipping and other conventional techniques. In its unfired condition, the thus-coated ware can be considered as being in the bisque condition. The conventional twofire system is to make the body, fire it at some high temperature, cool it, then apply the glazing material and refire at a lower temperature than that at which the body was fired. Oher practices include that of coating the unfired (green) body and firing the glaze and body simul- 1' taneously. In some instances, manufacturers have found it desirable to form the body, fire it only todryness at some low temperature, then apply the glaze and fire the coated body at a higher temperature. Hence, for my purposes, the body can be preformed andfired, or green, or

merely dried before application of the glaze, although the two-fire system is used most generallyin the U.S., is the most demanding on compositions for glazing; and the for glazing should be at least as low as the body to prevent crazing, advantageously about 10 10 C. lower than the body; for compression glazing this ditferential most desirably is 20 10' C. to 40X10- C., the body being the higher in expansion. Accordingly, the particular glaze composition utilized here should be matched to the body in diflerential of coefficient of themal expansion, the differential being in any case measured at the intended use temperature or over the use temperature range of the resulting glazed ware. For cookware thermal shock resistance is most desired over the operating temperature range of said ware. For dinnerware, compression glazing most desirably is practiced for obtaining mechanical shock resistance at ordinary room temperatures and dishwashing temperatures. Selection of the instant glaze for the appropriate body to get a combination of thermal shock resistance and mechanical shock resistance is, of course, quite possible within the limits of this invention.

Minerals generally used in making up the bodies are Petalite, cordierite, zircon, sillimanite, low expansion fire clays and/or wollastonite. Most suitable bodies for the instant purpose are those containing at least about 25% by weight of a lithia-bearing ceramic mineral such as petalite or a body containing at least about 50% cordierite so that the body has desired low expansion properties. Typical bodies can be compounded for high compression glazing having as a primary phase Petalite, or talc (to produce a substantial cordierite phase) or a zircon, or sillimanite (to produce a mullite phase) in combination with feldspars, clay, flint and/or silica. Bodies containing a high proportion of wollastonite also can be glazed suitably using glazes that fire at a temperature lower than that used in previously proposed glazing practice for this material (approaching 1200-1300" (3.), this being a definite advantage of applying the principles of the invention to wollastonite glazing practice.

Basically, to obtain the lowest expansion glaze, only enough flux is used to obtain the desired continuous vitreous surface which is adequately flowable into a coating film during the firing step, this minimum proportion of flux being as low as about for some of the lithiacontaining preparations, and at least about 7% in the most practical preparations described herein. The upper limitation of the flux content on the preparation is determined by the overall thermal expansion coeflicient of the composite glaze. Typical coeflicients of thermal expansion relative to flux content for the instant lithium-based preparations are as follows: 12% flux gives about 20x C.; 16% flux about 30 10-"/ C.; 20% flux about 40X10"/ C. The maximum flux content in any useful case with such preparations is about 30%, and in some instances it must be less to keep within the necessary limits of thermal expansion.

Raw materials for providing the ingredients of the preparations are those conventional in the glass makers art for supplying silica, alumina, and the other metal oxides for glass. The ingredients are provided alone or combined in various ways in minerals and chemicals. 'Purity of the raw materials mainly aifects color of the glaze, thus the purer raw materials are more versatile.

The flux can be a single material, but most generally is a mixture or combination for efliciency or economy. The composition for glazing can be made boron-free, but B 0 is advantageous to make the glaze most spreadable and fluent. Similarly, potassium oxide is desirable to promote gloss. The preferred flux is an approximately 50/50 mixture by weight of B 0 and K 0. The use of alkali metal oxides tends to raise the coeflicient of thermal expansion of the vitreous phase in the resulting glaze. Flux materials that can be used include: B O suitably in the form of borax, boric acid and/or calcium borate; K Osuitably in the form of potassium nitrate and/ or potassium carbonate; F-suita'bly in the form of calcium fluoride potassium silica fluoride, cryolite, sodium fluoride, and/or po- 10 tassium fluoride; PbO- suitably in the form of litharge and/or red lead (Pb O sodasuitably in the form of sodium nitrate, sodium carbonate, borax, feldspar and/or sodium fluoride; CaOsuitably in the form of whiting (calcium carbonate), calcium fluoride, wollastonite and/ or calcium feldspar; SrOsuitably in the form of strontium carbonate; and ZnOsuitably in this oxide form.

EXAMPLE 1 The following raw materials were weighed out and mixed in a twin-shell blender:

Parts Potassium nitrate 313 Boric acid 267 Lithium carbonate 441 Calcined alumina 446 Petalite 2129 The batch was placed in a fireclay crucible and smelted in an electric furnace at 1430 C. for 6 hours. It was then poured into water to frit it and the frit allowed to dry.

The frit then had the following composition:

Percent Si0 5 3.9 A1 0 27.0 Li 0 9.1 B 0 5.0 K 0 5.0

The frit was then dried and ball-milled for 16 hours. Subsequently, parts of the ball-milled frit, 0.2 part bentonite, 4 parts kaolin clay and 40 parts water were ball-milled together for one hour and the mixture sprayed onto a bisque-fired body of the following composition:

Percent Petalite 50 Potash spar 10 Ball clay 25 Kaolin 15 The body had been bisqued at 1260 and the coating was applied to a wet weight of one-half to one gram per square inch of body.

The glaze was then autocrystallized by firing the coated body as follows: heated at 500/hr. to 975; held one hour; cooled 180/hr. to 750; held one-half hour; cooled 180/ hr. to room temperature.

The result was a bright, high gloss, white, craze-free, continuous, non-porous, semicrystalline glaze coating on the tile body. The body itself had a coeflicient of thermal expansion of 17.7 x 10 C., and the corresponding coefficient of the coating was even lower in order to stay integral with such low expansion body.

EXAMPLE 2 The same kind of composition for glazing was sprayed onto the same kind of ceramic body as used in Example 1. The coated body then was fired at 1040 for 12 minutes in accordance with the Orth fast-fire process and apparatus shown in U.S. Pat. 3,322,413. The fired object was removed firom the fast-fire furnace and allowed to cool in air. The result was a high gloss tile with a smooth, crazefree, non-porous, translucent, semicrystalline glaze coating.

The batch was placed in a fireclay crucible and smelted in an electric furnace at 1430 C. for 6 hours. It was then poured into water to frit it and the frit allowed to dry. The hit then had the following composition:

Percent MgO 2 The frit was then dried and ball-milled for 16 hours. Subsequently, 100 parts ofv the ball-milled frit, 4 parts bentonite, and 45 parts water were ball-milled together for one hour and the mixture sprayed onto a bisque-fired body of the following composition:

Percent Uncalcined kaolin clay 36.7 .Talc 20.4 Barium carbonate 6.2 Calcined kaolin 36.7

The body had been bisqued at 1340 and the coating was applied to a weight of one-half to one gram per square inch of body.

The coated body then was fired as follows: heated at furnace rate (approximately 2 hours) to 1100; held 2 hours; cooled in the furnace (approximately hours) to i The batch was placed in a fireclay crucible andsmelted in an electric furnace at 1430 C. for 6 hours. It was then poured into water tofrit it and the frit allowed to dry.

The frit then had the following composition:

Percent Li O 9 2 22.5 SiO, 58.5 B203 K 0 5 The frit was then dried and ball-milled for 1-6 hours. Subsequently, 100 parts of the ball-milled frit and 65 parts of. water were ball-milled together for one hour and the v mixture sprayed onto a bisque-fired body of the following composition:

. Percent Wollastonite 55 Kaolin 30 Nepheline syenite The body had been bisqued at 1065 and the coating lwas applied to a weight of one-half to one gram per square inch of body.

11. EXAMPLE 3 p The following raw materials were weighed out and 7 mixed:

- Parts I Potassium nitrate 248 Boric acid 214 Lithium carbonate 44-1 Calcined alumina 446 Magnesia 61.2

Petalite 2129 The coatingon the body was then autocrystallized by firing as follows: heating at 500/hr. to 975; holding 2 hours; cooling at 180/hr. to 750; holding one hour; and cooling (furnace rate) to room temperature.

The result was a white, translucent, smooth, noncrazed,

non-porous semicrystalline glaze coating on the tile body.-

EXAMPLE 5 The following two batches were weighed out and mixed separately from each other:

Parts of- 313 314 267 0 622 453 0 223 Calcined alumina.-- 546 463 Petalite 1, 952 2, 036

Each batch was placed in a crucible and smelted in an electric furnace at 1430 for 6 hours. Each batch was then fritted in water and dried. The frits then had the following compositions.

Percent of- The frits'then were dried and ball-milled for 16 hours. Subsequently; 50 parts of milled Frit A and of milled Frit B, one-half part bentonite, and 60 parts water were ball-milled together for one hour and the mixture sprayed onto a body like that described in Example 1. The coated body then was fired in the manner described in Example 3.

The result was a white, opaque glaze of moderate gloss, 1'

free of crazing and non-porous.

EXAMPLE 6 A composition like A of Example 5 was prepared and fritted in the manner of Example 5. Then parts of this ball-milled frit, 5 parts of minus 325 mesh silica, 4 parts bentonite, and 60 parts of water were ball-milled together for one hour, and the mixture sprayed at the application weight of one-half to one gramper square inch onto a bisque-fired body (1260) of the following composition:

, Percent Petalite 56 Potash spar 10 Ball clay 22 Kaolin 12 The coated body was fast-fired at 1150 for one hour in the manner of Example 2 and allowed to cool-in air.

The result was a yellow, opaque glaze free of crazing, non-porous and glossy.

EXAMPLE 7 I The following raw materials were mixed:

Parts Boric acid 401 Calcined alumina 144 KNO Li CO 715 Petalite 2126 KzsiFs 149 The batch was placed in a fireclay crucible and smelted in an electric furnace at 1430 C. for 6 hours. It was then poured into water to frit it and the frit allowed to dry. The frit then had the following composition:

The frit was dried and ball-milled for 16 hours. One hundred (100) parts of it were mixed with 65 parts water and ball-milled for another hour. The mixture was sprayed onto a bisque-fired (1260") body at a weight of one-half to one gram per square inch of body. The body composition was:

Percent Petalite 30 SiO 15 Potash spar 1-5 Ball clay 25 Kaolin 15 The coated body was fired as follows: heated at 500/ hr. to 975; held one hour; cooled at 180lhr. to 700; held one hour; cooled at 180/hr. to room temperature.

The result was a glossy, translucent, uncrazed, nonporous glaze coating on the tile body.

EXAMPLES 8-18 Raw materials listed in Table I, below, were separately weighed out and mixed. Each batch was placed in a crucible and smelted and fritted as in Example 1. Each frit then had the composition given in Table H. Each frit then was dried and ball-milled for 16 hours. Subsequently, 100 parts of each frit were mixed with 4 parts bentonite and 40 parts water. Each such resulting slip was bill-milled for one hour more, then sprayed onto the bodies listed in Table III. Firing, crystal development and cooling to room temperature was done as in Example 3. The results are tabulated in Table IV.

14 EXAMPLE 19 The following raw materials were weighed out and mixed:

Parts -200 mesh silica 1088 Potassium nitrate 161 Boric acid 134 Magnesium carbonate 146 Barium carbonate 411 Magnesia 195 Talc 1414 The batch was placed in a fireclay crucible and smelted in an electric furnace at 1430 C. for 6 hours. It was then placed into water to frit it and the frit allowed to dry. The frit then had the following composition:

Percent MgO 8.36 BaO 10.55 A1 0 18.24 SiO 57.85 K 0 2.5 B 0 2.5

The frit was then dried and ball-milled for 16 hours. Subsequently, parts of the ball-milled frit, 4 parts bentonite and 40 parts water were ball-milled together for one hour and the mixture sprayed onto the body described in Example 1 at a weight of one-half to one gram per square inch of body.

The coated body was fired as described in Example 3.

The result was a brown, matte, craze-free, non-porous glaze on the tile body.

EXAMPLE 20 The same slip as described in Example 19 was prepared and milled. It was sprayed onto the body described in Example 3 at a weight of one-half to one gram per square inch of body.

The coated body was fired as described in Example 3.

The result was a cream white, matte, craze-free, nonporous glaze on the tile body.

TABLE I.RAW MATERIAL BATCHES Batch, parts Raw materials 0 D E F G H I J K L M O; 248 313 313 313 313 313 314 314 Boric acid. 214 267 267 111200 441 442 442 442 442 442 442 453 453 calcined alumin 446 446 446 446 446 446 446 463 311 etalite 2, 129 2, 129 2, 129 2, 129 2, 129 2, 129 2, 129 2, 036 2, 036 KzSiF; SrCO: 91. 5 N 3N0: 413 Mg() 153 Whitin 275 ZnO 0:10 BaCO Zirc 223 223 TABLE II.OXIDE COMPOSITIONS Batch designation 0 D E F G H I J K L M Wt. percent:

L1 0 9 9 9 g 9 9 9 9 9 9 27 27 27 27 27 27 27 27 27 22 54 54 54 54 54 54 54 54 54 54 4 4 5 5 g 4 5 5 5 5 5 5 5 TABLE III.-BODY COMPOSITION USED Example number Compoeition Same as Same as Same as Same as Same as Same as Same'as Same as Same as Same as Home as Ex. 1. Ex. 3. Ex. 1. Ex. 1. Ex. 1. Ex. 1. 7 Ex. 3. Ex. 3. Ex. 1. Ex. 3. Ex. 1.

TABLE IM-RESULTS I i I I i V Frit (from Table II) C D E E G H I I K L M t Body number (from Table III) ,s A s t Characteristics 8 9 10 11 12 13 V V 14 g 15 16 17 .18

- Gloss Moderate.. Glossy; Low gloss., Dead Dead Low gloss. Low gloss- Matte.- Matte.-." Low gloss. High f l m te. matte. v gloss. Opacity Opaque--- Clean..- Tzlans-t Opaque.-- Opaque Opaque--- Opaque--- 0paque Opaque.-. Opaque Opaque. ucen Color White Brown. White Creamy.- Whlte Creamy. Creamy Yellow... Creamy White .Whlte. Surface condition--- Smooth.-. Smooth. Smooth--- Matte Matte Smooth--- Smooth Somevghgt Matte Smooth-.- Smooth.

r e 1 Grazing No- No No No. No-.. No. No. No No.. No.. No. Porosity No No. No No.... No- No No- No No.. No... No. Average eoeflicient -8.0 11.0.- 19.9..-- 20.8 31.4---. 10.0 16.8.- 27.9-

of thermal expension over th range of 50- 1 350X10- C. v

EXAMPLE 21 25 Two glazing slips were i prepared by ball-milling each of the frits prepared above with water and bentonite by the method described in Example 21.

Each of these glazing slips were then applied and auto- To further demonstrate the criticality of the 17 weight percent alumina content, the following glazes were prepared and evaluated crystallized onto bisque fired 60% 'petalite bodies hav- Appropriate batch materials were separately smelted at ing coemciems of expansion of about C. The

1430 C. for 6 hours, fritted in water and dried to yield I bodies had been bisque fired by the method of Example the mt compositlpn set forth below" 4. In each test the result was a non-porous semicrystalline Percent otglaze which was very slightly crazed on the 60% petalite 21-A 21-,B 21-0 Y- Each of the glazing slips were then apphedand auto- 3 3 crystallized onto bisque fired 30% petalite bodies having 03 58.5 54 coefiicients of expansion of about 5 X 10-/ C. The Q g g bodies had been bisque fired by the method of Example 4.

1 In each test the result was a glossy, craze-free, non- Three glazing slips were prepared by ball-mllhng P porous, semicrystalline glaze on the 30% petalite body.

of the frits prepared above for 16 hours and subsequently 40 These two Series of tttstS (Examples Y and 22) demom furtherball-millmg 100 parts of each ball-milled fnt w1th strate the criticahty of the 17 weight ptent alumina com P 1 of Water and 4 Parts bcntonite- 1 tent in that these compositions are more suitable for glazglazing l were then applied and autocrystal ing bodies having a coeflicientof expansion of about rliud Onto bisque find bodies by the method of Example than for gazing bodies having a coefiicient of around 20.

Thc bisqlm fired bdie5 had a composition Similar 45 Having thus described the invention, what is claimed is:

y descfibcd in Example 1. In a process fonforming a substantially non-porous,

In each test; the result was moderate gloss craze-free," semictystalhhe glass eel-antic glaze on a fired ceramic non-porous, semicrystalhne glaze on the bisque body. body by the otwoyfire process, said glaze having a 81315 fi p y n bad a coemcient of P efficient of thermal expansion of less than 5 X IO-lf C.,

sion of about 49 l0"/ C. The glaze using frit 21B 50 the im rWement com risin the Ste of: had a coefiicient of expansion of about 42 10-"/ C. P g PS The glaze employing frit 21-C had a coetficient of expan- (a) at least partially coating said body with a glaze sion of about 21xl0-"/ C. Since alumina (at the excomprising ceramic frit consisting essentially of:

' use of silica) wasthe only variable in these series of rits, it is apparent that the alumina content is critical and fi g' Zigg alumina contents of 17 percent or more are required to '074 part of Mgo "plat? achieve glazing compositions having coeflicients of expanf' a 1 part U20 when M80 sion of less than 50X l0-7/ C. Y is used, but a minimum of i EXAMPLE 22 6o Al O :7 z being p n To further demonstrate the compositional criticality Sig): 3644; thc subtotal of of the present invention, the following glazes were prel M20, M80, A1203, and SK)2 pared and evaluated. being 80 95% Appropriate batch materials were separately smelted at z I about 1300 C for one hour, fritted in water and dried to Flux yield the frit'compositions (in weight percent) set forth below. wherein the subtotal of the ZI'Og, plus flux is 520% and wherein said flux is selected from the group 329921; a consisting of: B 0 K 0, F, PbO, Na O, CaO, SrO,

2 4k 22-13 h ZnO, BaO up to 5%, or a mixture ofsame,

(b) firing the resulting coated body at a temperature converting said composition into a fluent, continuous, vitreous surface coating; (c) then lowering the temperature to a value at which sufiiciently high and for a time sufiiciently long for 17 crystal growth in said vitreous surface coating occurs Within not substantially more than several hours; and

(d) then cooling the resultant glazed body at a rate consistent with keeping the body integral.

2. The process of claim 1 wherein: the firing is performed at a temperature not substantially more than about 500 C. above the liquidus of said hit; the temperature adjustment thereafter is downward to a temperature below the liquidus of the highest melting crystal produced in the vitreous matrix of said surface coating, but not substantially more than about 300 C. below the liquidus of the primary low expansion crystal phase formed by crystallization of said surface coating; and the cooling rate of the ware is not substantially in excess of about 40 C. per minute.

3. The process of claim 2 wherein said frit consists essentially of:

Ingredient: Percentage Li O 7-16.

MgO -2 /z, 0.74 part of MgO replacing 1 part of M 0, when MgO is used, but a minimum of 7% H 0 being present.

SiO 48-63, the subtotal sum of the Li O,

MgO, A1 0 and SiO being 88- 95%.

Flux 5-12.

and wherein the subtotal sum of ZrO plus flux is 5-12% and wherein said flux is selected from the group consisting of: B 0 K 0, F, PbO, Na O, CaO, SrO, ZnO, BaO up to 5%, or a mixture of the same.

4. In a fired ceramic body having a fired glaze frit coating thereon said body having been fired prior to the application of the glaze frit coating, said coating comprising a low expansion non-porous, semicrystalline glass ceramic glaze coating, said coating having a coeflicient of expansion of less than 5 l0 C. the improvement wherein said glaze frit consists essentially of:

wherein the subtotal of the ZrO plus flux is 5-20% and wherein said flux is selected from the group consisting of: B 0 K 0, F, PbO, Na O, CaO, SrO, ZnO, BaO up to 5%, or a mixture of same.

5. The article of manufacture of claim 4 which contains at least about 25% of a lithia-bearing ceramic mineral.

6. The article of manufacture of claim 4 which contains at least about 50% of cordierite mineral.

References Cited UNITED STATES PATENTS 3,499,787 3/ 1970 Inoue 117125 X 3,463,647 8/1969 Kosiorek et al. 117-125 X 3,404,027 10/1968 Kosiorek 117125 X 3,501,321 3/1970 Margola 117-125 X 3,532,524 10/ 1970 Petticrew 117-425 X 3,380,838 4/1968 Sack 10648 X WILLIAM D. MARTIN, Primary Examiner M. R. LUSIGNAN, Assistant Examiner U.S. Cl. X.R. 1 17169 A 

